Unbalanced fiber optic Michelson interferometer as an optical pick-off

ABSTRACT

A system for measuring changes in an environmental parameter, such as velocity or pressure, includes an optical signal source for providing a coherent light signal, and an interferometer having a first and second optical legs of unequal optical path lengths. The signal is split into first and second beams that are respectively directed into the first and second optical legs of the interferometer. A fixed mirror reflects the first beam received at the end of the first optical leg. An optical pick-off includes a movable mirror, positioned to reflect the second beam received from the end of the second optical leg. The movable mirror is movable in response to changes in the-value of the parameter to be measured. An optical coupler combines the first and second beams after they have been reflected back into their respective optical legs, producing an interference signal, which is detected by an optical detector. The detector generates an electronic signal having a value indicative of the value of the interference signal. The electronic signal is analyzed to correlate its value to changes in the value of the environmental parameter to be measured.

CROSS-REFERENCE TO RELATED APPLICATIONS

this application is a Division of application Ser. No. 09/114,582; filedJul. 13, 1998, now U.S. Pat. No. 6,317,213, which is acontinuation-in-part of application Ser. No. 08/848,090; filed Jun. 6,1997, now U.S. Pat. No. 5,949,740.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

The present invention relates to optical sensors used to measure changesin environmental parameters such as pressure, displacement,acceleration, and velocity. More specifically, the invention is directedto an optical pick-off as part of an interferometer that can be used insuch a sensor.

Michelson or Mach-Zehnder interferometers have been known for use incertain applications, such as acoustic sensors. A description of aMach-Zehnder interferometer used in an underwater acoustic sensor iscontained in, for example, U.S. Pat. No. 5,448,058 to Arab-Sadeghabadiet al.

An optical interferometer of known type includes a pair of opticalfibers into which a single source of light directs a light signal. Thelight signals, guided respectively through the two fibers, followoptical paths of different lengths, producing a phase difference betweenthe two signal beams when the beams are combined. The combined beams maybe detected by an optical detector. If the two signal beams have thesame polarization state when they are combined, the signals interfere toform a fringe pattern of bright and dark lines that is detected by theoptical detector.

Exposing either or both of the fibers to a change in the environmentalparameters, such as an acoustic pressure change, changes the fringepattern that is incident on the optical detector. Such changes in thefringe pattern as detected by the optical detector may be analyzed tomeasure the changes in the environmental parameters to which the fiberhas been exposed. In this manner, the nature of the acoustic waves towhich the fiber is exposed may be determined when the interferometer isused in an acoustic sensor.

Mach-Zehnder or Michelson interferometers employed in underwateracoustic sensor (“hydrophone”) systems use tens of meters of opticalfiber wrapped on a mandrel. The fiber is stretched and/or contracted toproduce a measured phase delay that is proportional to the changes inpressure resulting from acoustic waves. The interferometer has anoptical path length mismatch between its two optical legs that is on theorder of one meter, to allow the standard functioning and signalprocessing with a phase-generated carrier. See, for example, Kersey,“Distributed and Multiplexed Fiber Optic Sensors”, in Udd, Ed., FiberOptic Sensors: An Introduction for Engineers and Scientists, (New York,1991), pp. 347-363.

Fiber optic interferometric sensor systems, of the types describedabove, have found favor over piezoelectric hydrophone systems, due tosuch advantages as immunity to electromagnetic interference (EMI); theability to locate all electronic and electrical components and systemsin the towing vessel, rather than in the underwater environment; andenhanced capabilities for measuring vector quantities. The prior artfiber optic sensor systems, however, are relatively expensive tomanufacture. Thus, less expensive alternatives that provide the sameadvantages over piezoelectric systems have been sought. Batch-processedsilicon chip sensors, having a proof mass that is moved in response tochanges in environmental parameters, such as pressure and acceleration(which may result from, for example, vehicle or medium motion), havebeen employed as accelerometers and velocity sensors. Such siliconsensors are very inexpensive and quite rugged. Use of such siliconsensors in a hydrophone system, with the proof mass accessed by a fiberoptic delivery system, would lower costs as compared with prior artfiber optic systems. Making such chip sensors compatible with existingfiber optic architectures in Mach-Zehnder and Michelson interferometricsensing systems and the like has, however, proved troublesome inpractice.

It would therefore be a significant advancement in the state of the artto provide a fiber optic interferometric sensor system, in a hydrophoneor like application, that is capable of employing common,batch-processed silicon sensors.

SUMMARY OF THE INVENTION

The present invention is a measuring system that uses an inexpensivesilicon chip sensor in an optical interferometer to measure pressure,velocity, acceleration, or other environmental attributes or parameters.In a preferred embodiment, the sensor includes a movable proof mass thatis used as a movable mirror at the end of one leg of an interferometerhaving two unequal length legs. The proof mass of the silicon chipsensor moves in response to a change in a particular environmentalparameter, changing the optical length of the interferometer leg.

The present invention includes an optical signal source for providing apulsed, coherent it signal, and an interferometer having first andsecond fiber optic legs of unequal optical path length. The signal issplit into first and second beams that are respectively directed intothe first and second fiber optic legs. A fixed end mirror is placed onthe end of the first of the fiber optic legs for reflecting the firstbeam received at the end of the first fiber optic leg. An opticalpick-off is fixed beyond the end of the second of the fiber optic legs.The optical pick-off comprises a proof mass that is movable relative tothe end of the second fiber optic leg. The surface of the proof mass isreflective and positioned to reflect the second beam received from theend of the second fiber leg. An optical coupler combines the first andsecond beams reflected from the fixed end mirror and the proof mass,producing an interference signal. An optical detector optically coupledto the coupler detects the interference signal of the combined beams andgenerates an electronic signal having a value indicative of the value ofthe interference signal. The electronic signal is analyzed to correlateits value to changes in the value of the environmental parameter to bemeasured.

The measuring system of the present invention provides an accurate,relatively low-cost fiber optic interferometric sensor system, in ahydrophone or like application, that employs common, batch-processedsilicon sensors, and that is completely compatible with existing fiberoptic architectures of telemetric systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a measuring device constructedaccording to a preferred embodiment of the invention, incorporatingseveral interferometers;

FIG. 2A is a cross sectional view of an exemplary embodiment of asilicon chip sensor, of the type that may be employed as a velocitysensor, and one embodiment of a support block for holding the chipsensor and the end portion of the optical fiber in an optical pick-offconstructed according to the invention;

FIG. 2B is a top plan view showing a preferred embodiment constructionof the proof mass, supporting hinges, and frame of the silicon chipsensor of FIG. 2A;

FIG. 2C is an enlarged cross-sectional view showing an asymmetricalproof mass designed to compensate for gravity;

FIG. 3 is a cross-sectional view of the silicon chip sensor of FIGS. 2Aand 2B and a second embodiment of a support block for holding the sensorand the end portion of the optical fiber in an optical pick-offconstructed according to the invention;

FIG. 4 is a cross-sectional view of an alternative embodiment of asilicon chip sensor that may be used in the present invention;

FIG. 5 is a cross-sectional view of the silicon chip sensor of FIG. 2and a modified form of the first embodiment of the support block adaptedfor holding an optical fiber having an angled end face; and

FIG. 6 is an enlarged, detailed view of the angled end face of theoptical fiber and the ferrule of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in the context of itspreferred embodiments.

A measuring system 10, constructed according to the present invention,is illustrated in FIG. 1. The measuring system 10 comprises one or moreinterferometers that measure changes in the interference patterns inlight reflected from the ends of optical fiber paths having unequallengths.

Specifically, the measuring system 10 includes a light source 11, whichis preferably a laser producing an optical signal of coherent light inthe infrared or visible spectrum. The light source 11 may be directlypulsed, or it may be energized continuously, with its signal beingpulsed by a lithium niobate amplitude modulator 12, operated as anoptical gate, or by an equivalent mechanism. The pulsed signal is thenfiber-optically transmitted to a phase modulator 13, which creates aphase-generated carrier at a selected carrier frequency.

The pulsed and phase-modulated signal is propagated in a first directionthrough an optical fiber transmission line 15 to a single interferometeror a plurality of interferometers in series along the transmission line15. In the illustrated embodiment, three interferometers 31, 101, 121are shown, for purposes to be described below. For the purposes of theinstant discussion, only the first interferometer 31 will be described.

The first interferometer 31, which is optically coupled to thetransmission line 15 by a fiber optic down-link 36 and a first opticalcoupler 37, comprises a pair of unequal length optical fiber legs 33,35. The optical signal propagating through the transmission line 15 inthe first direction from the light source 11 is split into twointerrogation beams by a second optical coupler 38. The second opticalcoupler 38, which may be a conventional 3 dB optical coupler, directs afirst beam into the first leg 33, and a second beam into the second leg35.

The optical path length of the second optical leg 35 is substantiallygreater than optical path length of the first leg 33. For example, theoptical path length of the first leg 33 is as short as possible,preferably no more than about 10 centimeters in length. The optical pathlength of the second leg 35 may be as short as about one meter inlength.

As will be explained in detail below, the first and second beams arereflected at the ends of the first and second optical fiber legs 33, 35,respectively, returning through the optical legs to the second opticalcoupler 38, which recombines the reflected light signals for returnpropagation along the transmission line 15 in a second (opposite)direction to a photodetector 39. Changes in the relative optical pathlengths between the two legs cause changes in the interference patternsin the reflected light when it returns to the transmission line 15. Aswill be described below, and recognized by those skilled in the art, theanalysis of that interference pattern, and of the changes to it, permitschanges in the environmental parameters to which the interferometer 31is exposed (such as pressure and motion) to be determined.

The first optical fiber leg 33 (the shorter leg) has a normal highreflectivity end mirror 41 fixed at its remote, or second, end. Thisfixed end mirror 41 reflects the first beam propagated through the firstleg 33 back into the first leg in the second direction, toward the lightsource 11 (and toward the detector 39). The fixed end mirror mayoptionally be located on an immobile housing or frame (not shown).

At the remote, or second, end of the second fiber leg 35, an opticalpick-off 51 (of the type described in detail below) includes a siliconchip sensor containing a movable proof mass. The proof mass provides amovable end mirror that reflects the second beam propagated through thesecond leg 35 back into the second leg in the second direction, towardthe light source 11 and the detector 39.

The system shown in FIG. 1 is a time division multiplexing (TDM) system.Those skilled in the pertinent arts will recognize that the presentinvention may be employed in a frequency division multiplexing (FDM)system, and the modifications of the system shown in FIG. 1 needed toemploy the invention in an FDM system would be within the skill of suchpractitioners.

FIG. 2A is a cross-sectional view of a preferred embodiment of a singlesilicon chip sensor 53 that includes a movable proof mass 55. FIG. 2Bshows the general preferred configuration of a 9 mm by 9 mm proof mass55 supported at the corners by hinges 58 attached to a frame orperipheral mounting portion 57. Each hinge 58 has a width ofapproximately 500 microns. A peripheral mounting portion 57 has a lengthand width of approximately 12.5 mm by 12.5 mm. The proof mass 55 isseparated from the peripheral mounting portion 57 by a gap 49 ofapproximately 100 microns. The shapes and sizes of the proof mass 55,the gap 49, and the peripheral mounting portion 57 are exemplary, as arethe number and attachment sites for the hinges 58. Such a configurationis needed to prevent the proof mass 55 from distorting or undergoing acantilevered motion, which would cause the optical signal reflected offthe surface of the proof mass 55 to stray off the mark. The presentinvention sensor 53 operates with the proof mass 55 suspended in a fluidsuch as, for example, nitrogen or ambient air.

Referring now to FIGS. 2A and 2B, the optical pick-off 51 incorporates asilicon chip sensor 53 mounted on a pick-off support block 54. Siliconchip sensors suitable for the present invention are widely known andreadily available. They are relatively inexpensive, and may bemanufactured in large quantities. Silicon chip sensors use a movableproof mass to sense dynamic changes in the environment, such asvelocity, acceleration, or changes in pressure. An exemplary siliconchip sensor 53, as shown in FIG. 2A, includes an integral siliconelement comprising a movable proof mass 55 attached to a peripheralmounting portion 57 by means of flexible connecting portions or hinges58 along at least two opposed edges of the proof mass. The mountingportion 57 is securely fixed between a first housing portion 59 a and asecond housing portion 59 b, which define between them an internalhousing cavity 60 that contains the proof mass 55. The housing portions59 a, 59 b advantageously comprise plates of a ceramic material having alow coefficient of thermal expansion, preferably Pyrex® glass (marketedby Corning Glass, Corning, N.Y.), or an equivalent material. The proofmass 55 must be able to move within the cavity 60 in response to changesin environmental parameters, as will be described below. The secondhousing portion 59 b includes an opening 62, approximately aligned withthe center of the proof mass 55, through which the second optical beamis directed to the proof mass 55, as will be described below.

As mentioned above, the hinges 58 are preferably located at oppositeedges of the proof mass 55, thereby suspending the proof mass 55 atopposite ends. Suspending the proof mass 55 at its opposite ends ensuresthat it does not tilt relative to the incoming second optical beam in acantilevered fashion. Such tilting would skew the second optical beamthat is reflected off of the movable end mirror provided by the proofmass, so that the signal for the second optical leg 35 is lost.

To compensate for the effect of gravity on the proof mass 55, thepresent invention in a preferred embodiment incorporates asymmetry intothe profile of the proof mass 55. This is best seen in the enlargeddetailed view of FIG. 2C, showing a portion of the proof mass 55supported by a hinge 58 on the frame or peripheral mounting portion 57.The effect of gravity on the proof mass 55 is represented by downwardarrow g. To compensate for gravity, the proof mass 55 has an asymmetryon either side of the hinge 58, resulting in unequal distances l₁ andl₂. Prior to the effect of gravity, as depicted in FIG. 2C, l₁ is lessthan l₂ so that with the effect of gravity, the proof mass 55 shiftsdownward so that l₁ is equal to l₂. Gravity must be considered becausethe present invention is small.

The remote end portion of the second optical fiber leg 35 may becontained in a ferrule 63, which is preferably a ceramic tube. An axialpassage through the ferrule 63 holds the remote end portion of thesecond optical fiber leg 35, and is essentially the same diameter as thediameter of the optical fiber 35. The remote end of the ferrule 63, withthe remote end portion of the second optical fiber leg 35 containedtherein, is inserted into the opening 62 in the second housing portion59 b of the silicon sensor 53. A movable end mirror is provided by theproof mass 55 in the form of a highly reflective surface 64 that isspaced from and opposed to an end surface 65 of the second optical fiberleg 35 that lies substantially flush with the end surface of the ferrule63 within the opening 62. Preferably, this reflective surface 64 isprovided by a thin metallic coating, such as gold, to provide areflectivity close to 100%. Thus the light beam from the fiber 35 passesthrough air only as it exits the end face 65 of the fiber, is reflectedby the surface 64 of the proof mass, and returns to the fiber. The lightdoes not travel through other materials that may distort the beam.

The gap between the end surface 65 of the fiber 35 and the reflectivesurface 64 of the proof mass 55 should be sufficiently small that thereis minimal optical loss from beam spreading as the light exits the endof the fiber and is reflected back into the fiber. Preferably, the gapwidth is equal to at most a few wavelengths of the light propagatingthrough the fiber. For example, for wavelengths of interest, the gapwidth would preferably be between about 2.5 microns and about 20microns, so that the “round trip” distance the light travels in air isin the range of about 5 to about 40 microns. The end surface 65 of thesecond fiber leg 35 is coated with an anti-reflective coating (notshown) to minimize unwanted etalon reflections between the proof mass 55and the fiber end surface 65, and to ensure that all of the reflectedsignal enters the fiber. The anti-reflective coating on the end surface65, which also substantially eliminates retroreflection back into thesecond optical fiber leg 35, should have a reflectivity that ispreferably no more than about 0.1%. The fabrication of optical coatingswith such a low degree of reflectivity is known in the art, as shown,for example, in U.S. Pat. No. 5,529,671 to Debley et al., the disclosureof which is incorporated herein by reference.

The sensor 53 is mounted on the support block 54 so that the opening 62in the second housing portion 59 b coincides with an axial bore 74 inthe support block 54 that receives the ferrule 63. The sensor 53 isbonded to the support block 54 by an epoxy adhesive, preferably one thatis ultravioletcured. The remaining length of the second fiber leg 35outside the ferrule 63 may be contained within a typical fiber jacket(not shown), as is well known in the art. The pick-off support block 54may be annular, although the specific configuration and dimensions arematters of design choice to suit the particular application of concern.

Thus, the second optical beam passing through the second optical fiberleg 35 emerges from the end surface 65 of the fiber, and passes throughthe air gap between the fiber end surface 65 and the reflective surface64 of the moving proof mass 55. The reflective surface 64 reflects thelight beam back into the second optical fiber leg 35.

The optical pick-off using a silicon sensor can be used to measurevelocity, displacement, or acceleration, or changes in pressure. As isknown to those skilled in the art, acceleration is detected when theproof mass 55 of the sensor 53 moves within the cavity 60, and thusrelative to the housing portions 59 a, 59 b. Because the housingportions 59 a, 59 b are fixed with respect to the second optical leg 35,movement of the proof mass 55 within the cavity 60 also is movement withrespect to the second optical leg 35. Such movement changes the width ofthe optical gap between the reflective surface 64 of the proof mass 55and the end surface 65 of the second optical leg 35. The inner surfaceof the first housing portion 59 a is spaced a small distance from theproof mass 55, to permit the proof mass 55 to move in a single axiswithin the cavity 60. The entire structure may be contained in aneutrally buoyant housing (not shown).

Referring to FIG. 3, a modified pick-off 51′ having an alternatepick-off support block 82 is shown for use in applications in which asubstantially flat structure for the pick-off 51′ is necessary ordesired. The pick-off support block 82 receives the remote end of thesecond optical leg 35 and provides an optical path for the secondoptical beam between the end of the second optical leg 35 and the proofmass 55 of the silicon sensor 53. The pick-off support block 82 ispreferably formed of a disc-shaped piece of ceramic material, or amaterial of substantially equivalent thermal stability (i.e., lowcoefficient of thermal expansion), although its configuration anddimensions are largely matters of design choice, depending on theapplication.

The remote end portion of the second optical leg 35 is contained in aferrule 83, which, like the support block 82, should be formed of amaterial that has a negligible thermal coefficient of expansion, such asa suitable ceramic. The ferrule 83 is inserted into a first lateral bore85 in the support block 82, with sufficient clearance to allow theferrule 83 to be axially rotatable within the bore 85 so that theorientation of the light beam may be adjusted for optimal operation, aswill be described below. Furthermore, as will be seen, the ferrule 83 isalso preferably installed in the bore 85 so as to be axially movabletherein. An axial passage through the ferrule 83 holds the remote endportion of the second optical fiber leg 35. The second optical fiber leg35 has an end face 86 that is preferably angled about eight degreessubstantially to eliminate retro-reflection back into the second opticalfiber leg 35, without the need for an anti-reflective coating. For easeof manufacture, the end face of the ferrule 83 is similarly angled so asto be substantially flush with the second fiber leg end face 86.

A cylindrical graded index lens (GRIN lens) 87 is optically aligned withthe end of the second fiber leg 35 to focus a light beam emerging fromthe second optical leg 35 onto the reflective surface 64 of the proofmass 55. There is preferably a gap of approximately 0.2 mm between theend face 86 of the second optical fiber leg 35 and the facing opticalsurface of the GRIN lens 87. The GRIN lens 87 is contained in a secondlateral bore 89 in the support block 82 that is coaxial with the firstlateral bore 85.

The sensor 53 is bonded to the surface of the support block 82 so thatthe optical opening 2 in the second sensor housing portion 59 b isaligned with an air-filled optical passage 91 formed axially through thesupport block 82. To accommodate the ferrule 83, the GRIN lens 87, andthe components to be described below, the passage 91 is not necessarilycentered in the block 82.

A mirror rod 93 is installed in an eccentric bore 94 in a tubularfitting 95 which, in turn, is installed for axial rotation in a thirdlateral bore 97 in the support block 82. The inner end of the mirror rod93 terminates in a mirrored surface 99, cut at a 45° angle, thatprotrudes into the passage 91 so as to receive a light beam emergingfrom the GRIN lens 87. The second optical beam emerging from the end ofthe second optical fiber leg 35 propagates through the GRIN lens 87, isthen reflected at a 90 degree angle by the mirrored surface 99 on theend of the mirror rod 93, and then propagates through the passage 91.The tubular fitting 95 is rotatable within the third lateral bore 97 sothat the impingement point of the light beam on the mirrored surface 99may be adjusted for optimal operation, i.e., to minimize losses andunwanted reflections. An additional degree of adjustability may beobtained by installing the mirror rod 93 in the tubular fitting 95 so asto be axially rotatable within the eccentric bore 94. Additionalalignment adjustments can be effected by changing the distance betweenthe end of the second optical fiber leg 35 and the GRIN lens 87. Forexample, a coarse adjustment can be effected by moving the ferrule 83axially within the bore 85, and, due to the angled end surface 86 of thesecond optical fiber leg 35, a fine adjustment can be effected byrotating the ferrule 83.

The optical opening 62 in the second sensor housing portion 59 b isaligned so that the light path does not pass through the housingmaterial. Since the opening 62 registers with the passage 91 in thepick-off support block 82, the sec optical beam propagates only throughair once it exits the GRIN lens 87. Thus, the second optical beampropagating through the second optical fiber leg 35 emerges from the end86 of the fiber, passes through the air gap between the fiber end 86 andthe GRIN lens 87, and enters the GRIN lens 87. The GRIN lens 87 imagesthe light beam from the fiber end 86 onto the reflective surface 64 ofthe proof mass 55. The mirrored surface 99 reflects the beam ninetydegrees, causing the light beam to pass through the opening 62 in thesecond housing portion 59 b of the sensor 53, so as to impinge on thereflective surface 64 of the proof mass 55. The reflective surface 64reflects the light beam back toward the mirrored surface 99. Themirrored surface 99 reflects the reflected beam ninety degrees back intothe GRIN lens 87. The reflected beam then passes through the GRIN lens87 and re-enters the second optical fiber leg 35. Efficient coupling ofthe light beam from the fiber 35 to the proof mass surface 64 and backis important to obtain maximum effectiveness of the device. There shouldbe minimal back reflections at the fiber end 86 or elsewhere.

The mirrored surface 99 should be adjusted so that the reflected lightfrom the reflective surface 64 propagates exactly along the same path asthe beam impinging on the reflective surface 64. Thus, the mirroredsurface 99 should direct the beam as close to the center of the proofmass 55 as possible if the proof mass 55 flexes at all during use.

For applications in which changes in the environmental pressure are tobe measured (such as a hydrophone system), the structure of the sensormay differ from that shown in FIGS. 2A, 2B, 2C, and 3. As will beunderstood by those skilled in the art, in such an application the proofmass structure 55, 58 shown in the drawings would be omitted. Thestructure of such an alternative sensor 53′ is shown in FIG. 4. Thesensor 53′, which may be used with either the support block 54 of FIG.2A or the support block 82 of FIG. 3, includes a silicon diaphragm 100instead of a proof mass. The diaphragm 100 has a peripheral rim 102surrounding a flexible central area that flexes in response to changesin environmental pressure. The peripheral rim 102 is attached to a rigidbase plate 104, forming an internal optical cavity 60′ between thediaphragm 100 and the base plate 104. The base plate 104 has an opticalopening 62′ for the passage of the second optical beam, which strikes areflective surface 64′ that is applied directly to the interior surface(which faces the cavity 60′) of the diaphragm 100. Pressure changescause the central area of the diaphragm 100 to move, changing the pathlength of the optical gap defined by the width of the internal cavity60′.

An advantage of the pick-off 51 of FIG. 2A is that active opticalalignment or adjustment, if desired, may be effected simply bypermitting movement of the ferrule 63 axially in the bore 74. Thedisadvantage is the need for an anti-reflective coating on the end face65 of the second optical fiber leg 35. Conversely, the pick-off 51′ ofFIG. 3 eliminates the need for an anti-reflective coating, but activeoptical alignment requires adjustment of both the ferrule 83 and themirror rod 93. It would be advantageous to provide a pick-off in whichthe active optical alignment is simple, and that does not require ananti-reflective coating on the end face of the second optical fiber leg35. FIGS. 5 and 6 illustrate a modification of the first embodiment ofthe support block that accomplishes this result.

In FIGS. 5 and 6, a pick-off 51″ includes a support block 154 that isadapted to hold a second optical fiber leg 35′ with an angled endsurface 86′. A silicon chip sensor 53″, which may be similar to thesensor 53 described above and illustrated in FIGS. 2A and 3, is bondedto the support block 154. (Alternatively, a sensor similar to the sensor53′ of FIG. 4 may be used with the support block 154.) The chip sensor53′ has a proof mass 55′ with a highly-reflective mirrored surface 164,as decribed above. The support block 154 differs from the support block54 described above and illustrated in FIG. 2A in that the former has abore 174 that is offset from the normal to the plane of the mirroredsurface 164 by an angle α. In a preferred embodiment, the value of α isapproximately 3.7°, although it may range from about 3.4° to about4.00°. The bore 174 receives a ferrule 163 that contains the remote endportion of the second optical fiber leg 35′. The bore 174 communicateswith an opening 162 in a second housing portion 159 b′ of the sensor53″. The diameter of the opening 162 accommodates the entry therein ofthe remote end of the ferrule 163 at the angle α. Thus, the axis of theremote end portion of the second optical fiber leg 35′ is oriented atthe angle α with respect to the normal to the plane of the mirroredsurface 164.

Referring to FIG. 6, the second optical fiber leg 35′ comprises a core200 surrounded by a coaxial cladding 202, as is typical of conventionaloptical fibers. As mentioned above, the optical fiber 35′ has an endface 86′ that is cut at an angle. This angle, labeled θ in FIG. 6, isdefined as the angle between the end face 86′ and an imaginary line thatis perpendicular to the axis of the fiber 35′. The value of θ preferablylies in the range of approximately 8° to approximately 15°, depending onthe degree of reduction in retro-reflection desired and the type ofoptical fiber employed. Since the core 200 is centered in the cladding202, and since it is necessary to make the face of the core 200continuously planar, the angled end face 86′ must be cut so that thevertex of the angle is in the cladding 202 on one side of the core 200.The end face 86′ is preferably cut so that the angle θ is defined onboth s of the vertex. For ease of manufacture, the remote end of theferrule 163 is cut at the angle θ so that the perimeter of the end face86′ of the optical fiber 35′ is flush with the remote end surface of theferrule 163. Because the axis of the ferrule 163 (and therefore the axisof the optical fiber 35′) is offset at the angle α from the vertical (asoriented in the drawings), the half of the end surface 86′ of theoptical fiber 35′ that includes the core 200 is offset from thehorizontal by an angle having a value of θ+α, while the other half ofthe optical fiber end surface 86′ is offset from the horizontal by anangle having a value of θ−α.

The angled configuration of the end surface 86′ of the optical fiber 35′substantially eliminates retro-reflection back into the optical fiber ofthe optical beam emerging therefrom, while also minimizing unwantedetalon reflections between the mirrored surface 164 of the sensor 53″and the end face 86′ of the optical fiber 35′. The ferrule offset angleα compensates for the refraction, in accordance with Snell's Law, of theoptical beam emerging from the optical fiber 35′. Thus, the ferruleoffset angle a is selected so that the refraction of the optical beam atthe fiber/air interface (i.e., the fiber end surface 86′) results in anoptical beam that impinges on the mirrored surface 164 substantiallynormal to the surface thereof, so that it is precisely reflected backinto the optical fiber 35′.

Active optical alignment of the pick-off 51″ described above can beeasily and simply accomplished by adjusting the distance between themirrored surface 164′ and the end face 86′ of the optical fiber 35′.This adjustment may be made, for example, by moving the ferrule 163longitudinally in the bore 174.

In a sensor system constructed in accordance with a preferred embodimentof invention, the first and second optical beams are recombined andprocessed as follows: Referring again to FIG. 1, the second optical beamreflected from the pick-off 51 (which may be any of the embodimentsdescribed above) is propagated back in a second direction through thesecond optical fiber leg 35, while the first optical beam, reflectedfrom the fixed mirror 41, is propagated back in a second directionthrough the first optical fiber leg 33. The first and second opticalbeams are recombined in the second optical coupler 38 so as to form aninterference signal that changes with the motion of the movable mirrorof the pick-off 51 in response to changes in the value of theenvironmental parameter. The interference signal is propagated throughthe down-link 36 and coupled to the transmission line 15 by the firstoptical coupler 37. The interference signal is propagated through thetransmission line 15 and is transmitted to the photodetector 39 by meansof an optical fiber link 75 coupled to the transmission line by anoptical coupler 73.

Because the second optical beam reflected from the pick-off 51 hastraveled a different path length than has the first optical beamreflected from the end mirror 41, the light reflected from the pick-off51 interferes with the light reflected from the end mirror 41, creatingan interference pattern that changes as the proof mass 55 moves inresponse to the changes in the environmental parameter. The interferencepattern changes manifest themselves in changes in the value of theinterference signal detected by the photodetector 39, which generates anelectrical output signal having a value that indicates the changes inthe interference signal value. This electrical output signal is input toa microcomputer 77 (after suitable and conventional signal conditioningand digitizing) that processes the electrical signal, by means wellknown in the art, to correlate changes in the value of the interferencesignal with changes in the value of the environmental parameter, therebyyielding measurements indicative of changes in the value of theparameter.

In use of the invention as an accelerometer, movement of the body towhich the optical pick-off 51 is attached causes the proof mass 55contained in the housing 59 a, 59 b to move within the sensor cavity 60.That movement of the proof mass 55 changes the length of the opticalpath for the light propagating through the second leg 35. Thus, changingthe length of the optical path changes the interference pattern in thereflected interference signal that is detected by the photodetector 39.From such changes in the interference pattern in the reflected light,the movement of the proof mass 55 can be determined.

The invention may also be used as a pressure sensor, as in a hydrophone,preferably employing the sensor 53′ shown in FIG. 4. Changes in pressure(such as sound waves passing over the pick-off 51) cause the flexiblediaphragm 100 to flex, changing the length of the optical path for thelight propagating through the second leg 35. Thus, changing the lengthof the optical path changes the interference pattern in the reflectedinterference signal that is detected by the photodetector 39. From suchchanges in the interference pattern in the reflected light,environmental pressure changes can be measured. From such measuredpressure changes, information about sound waves causing those changescan be obtained.

Unlike the prior art, the optical fiber components in the presentinvention do not perform a sensor function. Sensing is performed throughthe silicon sensor 53 of the pick-off 51. For sensing environmentalchanges having a frequency above a few tenths of one Hertz, phase delaysin the optical fiber legs 33, 35 due to fiber stretching are negligible.

A phase generated carrier necessary for the described system requires asufficient optical path length mismatch between the two fiber legs 33,35. Current state of the art with respect to passive fiber optic sensorarchitectures dictates an optical path length mismatch between the firstand'second fiber legs 33, 35 that should be about 10 cm to about 1meter. Such an optical path length mismatch is also compatible withcurrent, state-of-the-art, stable, narrow line width laser sources andfiber optic architectures using internal frequency modulation and timedivision multiplexing or external phase modulation and frequencydivision multiplexing.

The optical pick-off 51 measures the displacement of the proof mass 55with respect to the fixed silicon sensor housing 59 a, 59 b atfrequencies above some minimum around one to five Hertz. While theoptical path lengths within the fiber legs 33, 35 may slowly drift overtime and temperature, such changes cause errors so low in frequency thatthey can be ignored for the purposes of making measurements with therequisite degree of accuracy. For example, a one degree Celsius changein temperature in one minute may produce fringe motion corresponding to0.1 Hz, far below the abovementioned minimum, when the difference inlength between the two fiber legs 33, 35 is approximately one meter.

For use as an accelerometer, the proof mass 55 of a given opticalpick-off 51 has a response in one direction only. Three interferometricsensors, each containing its own silicon chip sensor, may be used tomeasure motion in the three axes x, y, and z. A triad of such sensorsmay be mounted on one block.

FIG. 1 shows an exemplary embodiment of a system incorporating threesensors, such as might be used for measuring velocity or acceleration inthree directions. The second and third interferometers 101, 121 used asaccelerometers are substantially identical to the first interferometer31, although they may be any of the embodiments of FIGS. 2A, 3, or 5. Itis not necessary for all the interferometers in a particular system tobe of the same embodiment, nor is the system limited to any particularnumber of interferometers.

In FIG. 1, the second interferometer 101 contains unequal length fiberlegs 103, 105. One fiber leg 103 is substantially shorter than the otherfiber leg 105, the two legs being optically coupled by an opticalcoupler 106. The shorter leg 103 terminates in a fixed end mirror 107.The longer leg 105 terminates in an optical pick-off 109 that isadvantageously substantially identical to any of the optical pick-offs51, 51′, or 51″ described above. The second interferometer 101 iscoupled to the transmission line 15 by a fiber optic down-link 111 andan optical coupler 113. Similarly, the third interferometer 121 containsunequal length fiber legs 123, 125, joined by an optical coupler 126.The first fiber leg 123 is substantially shorter than the second fiberleg 125. The shorter leg 125 terminates in a fixed end mirror 127. Thelonger leg 125 terminates in an optical pick-off 129. The thirdinterferometer 121 is optically coupled to the transmission line 15 by afiber optic down-link 131 and an optical coupler 133. Additionalinterferometers of the same construction may be added to the system byoptically coupling them to the main transmission line 15.

For use in a hydrophone, a large number of interferometers (employingsensors 53′, as shown in FIG. 4) may be arranged in an array to be towed

behind a vessel. With sufficient laser source power dozens of thesedevices can be driven by one laser. With distributed gain from erbiumdoping in selected portions of the optical fiber, hundreds of theseinterferometers can be driven by one optical pump and one signal laser.

As described above, the present invention can be applied in adisplacement sensor, a velocity sensor, or an accelerometer. A properselection of proof mass dimensions, gaps between the proof mass and thesurrounding substrate, mass of the proof mass, the natural frequency ofthe suspension system, and fluid viscosity filling the gaps would yieldthe sensor of choice.

For seismic measurements, and assuming all other factors being equal, adisplacement sensor has good performance bandwidth, better than avelocity sensor, but requires a large gap and highly viscous fluids fordamping. The sensor must be large and bulky. An accelerometer has a verynarrow usable bandwidth. A velocity sensor has better bandwidth than anaccelerometer, with smaller gaps than in a displacement sensor, andoperates in air. Therefore, a velocity sensor would appear to be moreversatile than an accelerometer or a displacement sensor for seismicapplications.

Empirical studies have shown that for measuring displacement, velocity,or acceleration, the mass and proportions of the proof mass influencethe signal-to-noise ratio of the output signal of the sensor. Forexample, in a seismic sensing system incorporating ten or moreinterferometers on a transmission line similar to that shown in FIG. 1,it is preferable to have the output signal to be great enough inmagnitude to be discernible over optical and electrical noise, and yetstill operate within a useful frequency range of about 10 Hz to 500 Hz,for example.

Moreover, empirical studies indicate that to measure displacement, thesensor should have a proof mass of about 1 to 3 grams, with a squareconfiguration of about 14 mm on each side. For velocity measurements, aproof mass of about 0.5 to 2 grams, and an area of about 12 mm×12 mm ispreferred. To measure acceleration, it is preferable to have a proofmass of about 0.05 to 0.25 grams and an area of about 6 mm×6 mm. Proofmasses of these sizes and proportions, with a surrounding frame orsupport, can be constructed from a flat silicon wafer of about 400-500microns in thickness, for example, which is then etched by laser, gas,photolithography, or other techniques that are well-known in the art.

The interferometric system of the present invention can be adapted forthe use of silicon chip sensors that are even smaller in size than thosedescribed above. An example of such a silicon microstructure chip sensoris disclosed in U.S. Pat. No. 5,503,285—Warren, the disclosure of whichis incorporated herein by reference.

In addition, in the preferred embodiment, the proof mass in a velocityor acceleration sensor should be suspended in a gaseous medium, such asair or nitrogen, having a viscosity of about 0.00018 dyne-sec/cm². In adisplacement sensor, the proof mass is preferably suspended in oilhaving a viscosity of about 0.16 dyne-sec/cm².

The present invention velocity sensor is constructed based on anapproach of selecting the appropriate proof mass, damping fluid, etc.,to diminish the acceleration and displacement components. To see this, astarting point is with the standard expression for the motion of a proofmass in an open loop system:

Mx+C(f)x+K(f)x=−Mv+T

where:

x=displacement of the proof mass relative to the mass housing

x=velocity of the proof mass

x=acceleration of the proof mass

M=mass of the proof mass

C(f)=damping factor of the system

K(f)=spring constant

v=housing velocity

T is the applied force required to hold the proof mass at null; in otherwords, it is the force needed to rebalance the proof mass. In an openloop system, T equals zero.

The mass M and spring constant K are selected so that the damping factorC is much greater than the square root of the product of K and M. Thus,the acceleration and displacement terms may be ignored in the equation.Then dividing through the equation by C leaves:

velocity x≈−(M/C)v or x≈−(M/C)v assuming C>>(KM)^(¼) where K≠0 and M≠0.

Hence, an open loop velocity sensor for seismic applications can beconstructed using the above formula. An exemplary embodiment velocitysensor could be constructed based on the following ranges in the band ofinterest:

The mass is 0.050<M<0.250 gm.

The spring rate is 100,000<K<5,000,000 dyne/cm.

The damping factor is 10,000<C<60,000 dyne/(cm/sec).

The foregoing velocity sensor can be adapted for use with the opticalpick-off described above. It is also contemplated that the presentinvention velocity sensor can be adapted for use with an electrostaticpick-off.

Although several preferred embodiments have been described herein, suchembodiments are exemplary only. A number of variations and modificationsmay suggest themselves to those skilled in the pertinent arts. Forexample, the configuration and dimensions of the support blocks 54 (FIG.2A), 82 (FIG. 3), and 54′ (FIG. 5) may be varied to suit differentapplications. Also, the above-described alignment adjustment mechanismfor the mirror rod 93 in the FIG. 3 embodiment may be modified to beadjustable in additional directions, or it may be omitted altogether.These and other variations and modifications are considered within thespirit and scope of the invention, as defined in the claims that follow.

What claimed is:
 1. An optical pick-off for an interferometric sensingsystem or the like, comprising: a housing having an optical opening; anoptical fiber having an end portion disposed in the optical opening, theend portion having an axis; and a movable mirror that moves in responseto changes in a specified physical parameter, whereby the movable mirroris disposed with respect to the end of the optical fiber so that anoptical beam emerging from the end of the optical fiber impinges uponthe movable mirror and is reflected from the movable mirror back intothe end of the optical fiber; the axis of the optical fiber end portionbeing oriented at an angle α with respect to the normal to the plane ofthe mirror.
 2. The optical pick-off of claim 1, wherein the angle α isbetween about 3.4° and about 4.0°.
 3. The optical pick-off of claim 1,wherein the movable mirror comprises a proof mass having a reflectivesurface.
 4. The al pick-off of claim 1, wherein the movable mirrorcomprises a pressure-responsive diaphragm having a reflective surface.5. The optical pick-off of claim 1, further comprising: a support blockon which the housing is mounted, the support block having a boreoriented at the angle α and extending to the optical opening; and afiber-holding member extending through the bore, whereby the end portionof the optical fiber is disposed within the optical opening.
 6. Theoptical pick-off of claim 5, wherein the fiber-holding member includes aferrule disposed in the bore, the ferrule having an axial passagecontaining the optical fiber, whereby the end portion of the opticalfiber is disposed within the optical opening.
 7. The optical pick-off ofclaim 1, wherein the end portion of the optical fiber terminates in anend face that is configured with an angle θ, wherein θ is defined as theangle between the end face and an imaginary line that is perpendicularto the axis of the fiber, and wherein the angle θ is between about 8°and about 15°.
 8. The optical pick-off of claim 1, wherein the supportblock has a bore aligned with the opening, and wherein the fiber-holdingmember includes a ferrule disposed in the bore, the ferrule having anaxial passage containing the optical fiber, whereby the end surface ofthe optical fiber is disposed within the optical opening.
 9. An opticalpick-off for an interferometric sensing system or the like, comprising:a housing having an optical opening; an optical fiber having an endportion disposed in the optical opening, the end portion having an axis;and a movable mirror comprising a proof mass having a reflective surfacethat moves in response to changes in a specified physical parameter,whereby the movable mirror is disposed with respect to the end of theoptical fiber so that an optical beam emerging from the end of theoptical fiber impinges upon the movable mirror and is reflected from themovable mirror back into the end of the optical fiber; the axis of theoptical fiber end portion being oriented at an angle α with respect tothe normal to the plane of the mirror.
 10. The optical pick-off of claim9, wherein the angle α is between about 3.4° and about 4.0°.
 11. Theoptical pick-off of claim 9, further comprising: a support block onwhich the housing is mounted, the support block having a bore orientedat the angle α and extending to the optical opening; an a fiber-holdingmember extending through the bore, whereby the end portion of theoptical fiber is disposed within the optical opening.
 12. The opticalpick-off of claim 11, wherein the fiber-holding member includes aferrule disposed in the bore, the ferrule having an axial passagecontaining the optical fiber, whereby the end portion of the opticalfiber is disposed within the optical opening.
 13. The optical pick-offof claim 9, wherein the end portion of the optical fiber terminates inan end face that is configured with an angle θ, wherein θ is defined asthe angle between the end face and an imaginary line that isperpendicular to the axis of the fiber, and wherein the angle θ isbetween about 8° and about 15°.
 14. The optical pick-off of claim 9,wherein the support block has a bore aligned with the opening, andwherein the fiber-holding member includes a ferrule disposed in thebore, the ferrule having an axial passage containing the optical fiber,whereby the end surface of the optical fiber is disposed within theoptical opening.
 15. An optical pick-off for an interferometric sensingsystem or the like, comprising: a housing having an optical opening; anoptical fiber having an end portion disposed in the optical opening, theend portion having an axis; and a movable mirror comprising apressure-responsive diaphragm having a reflective surface that moves inresponse to changes in a specified physical parameter, whereby themovable mirror is disposed with respect to the end of the optical fiberso that an optical beam emerging from the end of the optical fiberimpinges upon the movable mirror and is reflected from the movablemirror back into the end of the optical fiber; the axis of the opticalfiber end portion being oriented at an angle α with respect to thenormal to the plane of the mirror.
 16. The optical pick-off of claim 15,wherein the angle α is between about 3.40°and about 4.0°.
 17. Theoptical pick-off of claim 15, further comprising: a support block onwhich the housing is mounted, the support block having a bore orientedat the angle α and extending to the optical opening; and a fiber-holdingmember extending through the bore, whereby the end portion of theoptical fiber is disposed within the optical opening.
 18. The opticalpick-off of claim 17, wherein the fiber-holding member includes aferrule disposed in the bore, the ferrule having an axial passagecontaining the optical fiber, whereby the end portion of the opticalfiber is disposed within the optical opening.
 19. The optical pick-offof claim 15, wherein the end portion of the optical fiber terminates inan end face that is configured with an angle θ, wherein θ is defined asthe angle between the end face and an imaginary line that isperpendicular to the axis of the fiber, and wherein the angle θ isbetween about 8° and about 15°.
 20. The optical pick-off of claim 15,wherein the support block has a bore aligned with the opening, andwherein the fiber-holding member includes a ferrule disposed in thebore, the ferrule having an axial passage containing the optical fiber,whereby the end surface of the optical fiber is disposed within theoptical opening.